As public safety agencies and other organizations evaluate their two-way radio needs for the future, a number of possible solutions are available, including the digital radio technologies that are now available.
For instance, the Association of Public-Safety Communications Officials (APCO) Project 25 (P25) (or APCO-25) represents one effort to set standards for digital two-way radio technology. In general, P25 refers to a suite of narrowband digital Land Mobile Radio (LMR) communication standards for digital radio communications, equipment and systems. P25 standards are produced through the joint efforts of the Association of Public Safety Communications Officials International (APCO), the National Association of State Telecommunications Directors (NASTD), selected Federal Agencies and the National Communications System (NCS), and standardized under the Telecommunications Industry Association (TIA). P25 technology and products are not limited to public safety alone and have also been selected for other private system applications.
P25 standards specify a Common Air Interface (CAI) that defines the type and content of signals transmitted by P25-compliant radios. P25-compliant radios can communicate directly with each other in “talk around” mode without any intervening equipment between two radios, or in conventional mode where a requesting radio chooses the channel to talk on and two radios communicate through a repeater or base station without trunking. In addition, two radios may communicate in a trunked mode where infrastructure equipment dynamically assigns the requesting radio a channel to talk on and traffic is automatically assigned to one or more voice channels by a Repeater or Base Station. Further details regarding the P25 standards can be obtained from the Telecommunications Industry Association, 2500 Wilson Boulevard, Suite 300 Arlington, Va. 22201.
P25-compliant technology is being deployed in several phases commonly referred to as Phase 1 and Phase 2. Phase 2 is currently under development to improve spectrum utilization. Among other changes to the Phase 1 standard, the Phase 2 standard proposes significant changes to the CAI. One of the major changes is the migration from an FDMA-based channel access scheme to a TDMA-based channel access scheme.
In any wireless communication system, radio signals are subject to wide power level variations over time due to shadowing, fading, change in distance between a mobile radio and a base radio, etc., and thus exhibit signal quality variations with respect to the communication links used for communicating between them. As such, in any wireless communication network, it is important to provide a mechanism for infrastructure equipment to determine when a call (or communication session) has ended so that communication resources assigned to that call can used by other mobile stations.
In some wireless communication systems, when a mobile radio completes a transmission it can transmit an explicit termination instruction (sometimes called an end-of-call message) to infrastructure equipment. In response the infrastructure equipment terminates the call, and the communication channel and other resources may then be made available for other calls.
In a wireless communication network, any condition where the radio signal strength weakens to the point where radio communications are affected can be referred to as “fading.” Fading can be due to short term variations in radio signals (e.g., Raleigh fading), or due to an increase in distance between the transmitting radio and the receiving radio. For instance, in some cases a mobile station does not intend to end its call, but moves out of range of the base station that it is communicating with. Because the mobile station does not intend to end the call, an end-of-call message is not transmitted, and hence the infrastructure equipment (e.g., base station) does not receive an explicit end-of-call message from that mobile radio. In other cases, the mobile station might transmit an end-of-call indication, but it may not be received by the infrastructure equipment or might, for example, contain too many errors to be decoded correctly.
When such fade conditions occur, the infrastructure equipment should nevertheless be able to detect and confirm that the call has ended, and if so, terminate the call and free up any communication resources assigned to the particular mobile radio involved in that call. At the same time, because fading happens regularly in a wireless environment, it is desirable for the infrastructure equipment not to prematurely drop a call just because a temporary fade condition has been detected. In the event a call is prematurely dropped due to fading and the mobile station is still within the coverage area of the infrastructure equipment, the infrastructure equipment should also be able to resume the call as soon as possible.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.